The Creature from Into The Pit. It's an Animalistic Abomination living in an underground cave system that kills people and either eats them or adds their remains to it's 'nest'. It's invulnerable to heat and flame, and can apparently create passageways through rock at will.
Praplant's species, the Fuge Insidiae. Their method of hunting is to lie in wait among other vegetation for prey to step on one of their vines. They then quickly constrict their bodies, crushing their prey into an easily digestible paste. This wouldn't be so bad, considering all the various methods of hunting carnivorous plants on Earth have, except for the fact that Fuge Inisidiae are TWO STORIES TALL.
It's also worth noting that Praplant was Tech's only form that was able to kill the above Creature, and just so happened to accomplish this feat by eating it.
The Void. While not visually terrifying, The Void is still an Eldritch Abomination that happens to be the main cause of people going Ax-Crazy. What makes it worse is that it's apparently necessary for The Void to do this, though exactly what will happen if it didn't is not made clear.